This research consists of a novel intervention designed to increase physical activity of premature babies in their first year of life. The potential beneficial impact of augmented physical activity on 1) body composition, 2) associated biochemical and cellular mechanisms of growth and inflammation, and 3) quality of maternal care will be measured. Premature birth is recognized as the single most important health problem in maternal- child health in the US. Paradoxically, both failure to thrive and obesity are now known to be associated with prematurity, as are osteopenia and increased risk of fracture, and increased risk of cardiovascular disease later in life. We lack cohesive approaches to mitigate these profound threats to health. Despite promising new research demonstrating that physical activity can stimulate the growth of muscle and bone even during intrauterine life (perhaps through metabolic programming), there have been very few attempts to implement and study physical activity interventions in the premature baby. The challenges are substantial and include: measuring body composition; assessing physical activity; engaging caregivers as partners in the intervention; identifying plausible and testable biological mechanisms; and designing interventions that actually increase physical activity and match the rapid pace of motor development early in life. This team of investigators has addressed these challenges. A developmentally dynamic physical activity intervention has been designed and pilot tested-one that engages the caregiver as a partner. Using techniques and tools as far-ranging as DXA, smartphones, doubly labeled water, and lightweight, wireless accelerometers developed specifically for this purpose, the working hypothesis is that the one-year intervention will augment lean body mass (primary outcome variable) and improve bone mineralization and the ratio of lean to fat tissue (secondary outcome variables). The study will gauge the impact of the physical activity intervention on the balance between circulating anabolic mediators (insulin-like growth factor-I and growth hormone binding protein) and inflammation-associated cytokines (interleukin-6, and interleukin-1 receptor antagonist), which antagonize muscle and bone growth. We will additionally: 1) begin to explore how physical activity influences circulating endothelial progenitor cells, which are increasingly viewed as markers of vascular health very early in life, and 2) take advantage of this prospective, interventional study to explore potential genetic determinants of growth in babies born prematurely. Finally, any study involving the premature infant and the data associated with it must be viewed in light of the critical relationship between the mother and baby. The potential broad impact of early life interventions has been recently demonstrated by the success of the Back-To-Sleep campaign in mitigating sudden infant death syndrome. Should the positive effects of augmented exercise on body composition be supported, we would then anticipate widespread benefit in preventing long-term health consequences of prematurity at relatively low cost.